The 55 islands and the sparkling seas around them are famed for their clean waters and pristine coral reefs. They are described by naturalists as the "other Galapagos", "a lost paradise" and a "natural wonder" and are officially recognised as a biodiversity hotspot of global importance.

This week the British government, backed by nine of the world's largest environment and science bodies, including the Royal Botanic Gardens at Kew, the Royal Society, the RSPB and Greenpeace, is expected to signal that the 210,000 sq km area around the Chagos Islands in the Indian Ocean will become the world's largest marine reserve. If it does, all fishing, collection of corals and hunting for turtles and other wildlife will be banned across an area twice the size of the British isles.

More than 275,000 people from more than 200 nations have sent messages in support of Britain's full protection of the Chagos Islands and their surrounding waters, but one group is distinctly uneasy.

The original Chagossians, who were deported between 1967 and 1973 to make way for a giant US nuclear air force base on the largest island, Diego Garcia, say they would in effect be barred from ever returning because the marine protection zone would stop them fishing, their main livelihood. "There would be a natural injustice. The fish would have more rights than us," said Roch Evenor, secretary of the UK Chagos Support Association, who left the island when he was four.

The islanders, who number about 4,000 and live in exile in Britain, Mauritius and elsewhere, have battled through the British courts for nearly 20 years for the right to return and appeared to have won an important victory in 2000 when the then foreign secretary, Robin Cook, decided in their favour. But following the September 11 attacks, the UK government reversed Cook's decision and the Chagos case has migrated between courts. Most recently, the House of Lords ruled against them after Britain cited American security concerns. Their last hope is that the European court of human rights will overturn the decision in their favour in the next few months.

Today, Chagossian supporters accused the government of duplicity. "The British government's plan for a marine protected area is a grotesquely transparent ruse designed to perpetuate the banning of the people of Mauritius and Chagos from part of their own country," said Ram Seegobin, of the Mauritian party Lalit de Klas, in a letter to Greenpeace seen by the Guardian. "The conservation groups have fallen into a trap. They are being used by the government to prevent us returning," said Evenor.

They were backed by Clive Stafford Smith, director of the human rights group Reprieve, who has challenged the UK government on the use of Diego Garcia by the US to render suspected terrorists. "The truth is that no Chagossian has anything like equal rights with even the warty sea slug. There is no sense that the British government will let them go back. The government is not even contemplating equal rights for Chagossians and sea slugs."

Supporters of the islanders also suspect that the timing of the announcement of the protected area is highly political. "Clearly, the British government is preparing a fall-back plan; if they lose the case in Europe, then there will be another 'reason' for denying the banished people their right of return," said Olivier Bancoult, a Chagossian leader in Mauritius.

Today, scientists and conservationsists denied that they were being "used" by the government.

"The UK government agrees that a marine protection area will not create a barrier for the Chagossians to return. The two issues are separate. If the Chagossians are given a right to return, any conservation measures will be adjusted. The aim is to protect the reserve now so that the resources there would be available for the Chagossians if and when they return. As it is, the seas there are being heavily depleted by French and Taiwanese fleets," said a spokeswoman for the US-based Pew environment group, which is expected to contribute millions of dollars to establish the reserve.In a letter on its website, Greenpeace said: "[We] acknowledge and support the Chagossians in their struggle, and hope that they are successful. But at the moment, the Chagos Islands are being administered by the UK government, and whatever way you look at it, taking steps to protect the marine life there is a good idea. If and when the Chagossians are repatriated, then the protection of the seas around the archipelago will need to be readdressed, and yes, that may well involve allowing fishing by the islanders."

But David Snoxell, former high commissioner to Mauritius, said the marine reserve would set up a significant barrier to the Chagossians' return. "The environment groups were beguiled [into giving their support]. If the government were to designate a protection area they would be erecting a psychological, legal and economic barrier against the Chagossians, and send a strong message that they would not be welcome in their homeland. It would be highly prejudicial."